Semiconductor devices operative to emit light when activated are exemplified by light emitting diodes (LEDs). Such devices are used to form an array of devices or elements each of which produces a light output (or not) depending upon whether it is activated or not. It is known to use a linear array of LED elements to expose consecutive linear segments of an electrostatic drum commonly used in photocopiers. The LEDs discharge the consecutive linear segments of the drum as the drum rotates.
One problem with such an arrangement is that a linear segment across the drum requires 2048 LED elements and semiconductor chips have not been made in which such a number of LEDs can be formed in an uninterrupted line. Such a problem is not easily corrected. U.S. Pat. No. 4,435,064, for an invention of T. Tsukada et al., describes various arrangements of LEDs which produce the effect of an uninterrupted linear array of LEDs. In one arrangement, two lines of spaced apart LED chips are formed, and a plurality of chips is used in each line. The chips are arranged so that the LEDs appear in spaced apart groups where the groups in the second row align with the spaces in the first. The two lines of LEDs are aligned across the drum and when timed properly produce a continuous linear effect in discharging the drum. But this technique requires careful alignmentof the chips and relatively complicated and thus expensive control circuitry. Alternatively, LEDs may be arranged in groups where each group is aligned along an axis at 45.degree. with respect to the direction of rotation of the drum. The end diodes of each group are aligned adjacent to a line parallel to the direction of rotation. Again, when properly activated, the effect of a straight linear array across the entire drum is achieved.
Relatively short linear arrays of LEDs cannot, at present, be abutted to form a longer array because when the chips are cut, the spacing between the diode closest to the edge of the chip and the edge of the chip is larger than the spacing between adjacent LEDs. The main reason for this spacing problem is that chips are cut by a dicing wheel which has a bevel to its cutting edge. That bevel dictates a setback from the edge of the chips to the closest diode. Considerations as to damage caused by the dicing wheel dictate a further setback. Of course, when two identical such chips are abutted, the total spacing between the two diodes closest to the adjacent edges of these chips is twice the setback of each diode from the edge thereof; as a result there may be objectionable spacings in a linear array formed by abutting a number of chips having small spacings between diodes for high density applications. Although in low density applications abutted conventional arrays may be usable, the arrangement poses problems for printing heads where close spacings are required.